This invention relates to seismic exploration and more particularly to a method for improving velocity determinations from the recorded seismic data by correcting for static time shifts between seismic traces.
In seismic exploration, seismic energy is generated by a source located at or near the surface of the earth, is reflected from subsurface interfaces between layers of the earth, and is received by a spread of seismic receivers on the surface of the earth. The receiver signals are conventionally gathered, stacked, and recorded in the form of a suite of common depth point (CDP) seismic traces as illustrated in FIG. 1. From these recorded CDP seismic traces the subsurface interval velocities can be determined. Such a CDP data gathering and recording technique and a continuous velocity determination technique are disclosed in U.S. Pat. No. 3,651,451 to William H. Ruehle. An accurate velocity determination is quite important in the interpretation of the nature and extent of the subsurface layering.
Two time corrections must be applied to the received seismic signals before they are stacked and recorded as CDP traces. First, each of the traces must be corrected for what is commonly referred to as normal moveout. The traveltimes of the seismic pressure wave from the seismic source to the seismic receivers differ in accordance with the differing spacing of the receivers from the source. To correct for this, a normal moveout correction which is related to the distance from the source to each receiver is applied to the seismic signal recorded by each such receiver.
Second, the top portion of the earth's crust, referred to as the surface layer, has varying thickness and velocity characteristics from place to place which introduce an unknown time shift into each recorded seismic signal. Such a time shift is referred to as a "static error" or "static time shift," and the correction necessary to correct a seismic signal for such an error is called "static correction" or more simply "statics."
Static errors can occur in either a surface-consistent fashion or in a nonsurface-consistent fashion. Where the surface layer is generally shallow or thin, the static shifts along the seismic traces are relatively constant from trace to trace. In this case, the traces can be shifted by a constant time increment to a common datum plane below the surface layering so as to make the traces appear to have been recorded at detectors located on the datum plane in response to acoustic energy generated by a source located on the datum plane. However, in those areas where the surface layering is deep or thick and with variable thickness or velocity, the static errors are not consistent from trace to trace.
The present invention is particularly useful in enhancing the velocity determination of the recorded seismic data in the presence of a deep and variable thickness surface layering by providing for a method for determining the nonsurface-consistent static errors created by such surface layering and applying time variant shifts to the received seismic signals to correct for such nonsurface-consistent static errors prior to CDP stacking and trace recording.